3.07.2021 Shutter slides clearances resolved.

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Saturday 3rd 64F, heavy overcast with light showers.

First I made the slit ribs parallel and pressing outwards against the cut edge of the slit. I used a ratchet strap to pull the cut edges of the slit together to match the spacing at top and bottom. Acetone quickly removed the magic marker cut lines I no longer needed.

My confusion over the minimum height of the [curved] shutters has finally been resolved. I screwed the shutter slides to a sturdy batten to bridge the slit cut-out. This allowed only minimum slide clearance above the crest of the dome profile. Once that was established it was obvious how much clearance there was for the outer edge of the [downward curving] shutter profiles.

The first image shows the slides on the inside of the dome for clarity. With the dome profile board sitting on top. This fixes the minimum height of the shutters when closed. There has to be clearance under the outer edges of the shutters for the slides. 

The 2x4 in the foreground is just a brace to hold the slit ribs firmly apart. [93cm between the ribs] It also acts as a safety rail. For when I stand on the stepladder inside the dome.

The second image shows the positions of the slides with the shutters closed. Here the slides are facing outwards towards the back of the dome. The red arrows show the vital clearance points. The faceted dome profile has a raised crest. Which is a little higher than a simple hemisphere [red curve.]

 

The third image shows how the shutter profiles will appear when open. There will be closing boards at the rear of the shutters. To weather-protect the slides. So the slides will be hidden from view.

I have painted in the shutter profiles in bright green for contrast against the dark background. There will be plywood ribs on each long edge of the shutters.

Later I cut a new, full depth, dome profile board from 12mm plywood. Then fitted the shutter slides to the board. 

This clearly shows the minimum height for the slides and shutters. I had hoped for less projection but it seems impossible without introducing a slope to the slides. Which might open up an unforeseen can of worms.

More of these full depth boards, as glued laminations, will help to spread the loads into the top of the dome. Internal profile boards will fit under the top of the dome. The final, inner "zenith board" will be an aluminium plate covering both the upper and lower, profile board sandwich. I shall fit large eye-bolts here for my OTA handling, rope and pulley system.

Narrower, slit ribs will be cut to rest on top of the dome along the edges of the observation slit. These will close off the butt joint between the slit ribs and the dome. They will also help to spread the loads into the dome over the entire length of the slit ribs. 

Silicone sealer will be applied at the joint. Then more silicone sealant between these narrower ribs. With more silicone between the sealing ribs the dome itself as they are lowered into place. I could have used resin and matt at the joint but decided it would be much neater to use a mechanical cover in the form of a narrow rib. GRP is messy and untidy and rather permanent. It is very hard to undo such a joint without damage.

I have a box of stiff angle brackets waiting to secure the slit ribs on the inside of the dome. The mushroom head, fixing bolts will have rubber, weather sealing gaskets [washers] under their heads. This should seal them against water penetration.

I am thinking of mass producing these washers using sharp-edged pipe in the drill press. This will [hopefully] be easier than trying to punch them out. Which didn't go well. The hole punch just bounced off the rubber! A suitable drilling lubricant might be washing up liquid and water. Or cooking oil? I have lots of heavy duty rubber sheeting [pond liner] scraps left over from the plywood dome skirts.

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